1. Field of the Invention
The present invention relates to method and apparatus for substantially reducing cross-polarized radiation in offset reflector antennas and, more particularly, to method and apparatus for substantially reducing cross-polarized radiation in offset reflector antennas by disposing a polarization grid comprising a plurality of nonparallel spaced-apart elements derived from a family of hyperbolae between a main curved focusing reflector and an associated feedhorn.
2. Description of the Prior Art
Cross-polarized radiation from an offset reflector is often regarded as a blemish on an otherwise excellent antenna which offers both low sidelobe level and good impedance matching. Although the cross polarization can be minimized using a large effective F/D ratio, the corresponding requirements of small offset angle and large feed aperture are not always convenient in applications.
Various techniques have been devised to reduce cross-polarized radiation in a transmitted or received beam, where the cross-polarized radiation is introduced by various elements encountered by the beam. For example, U.S. Pat. No. 3,914,764 issued to E. A. Ohm on Oct. 21, 1975 relates to apparatus for reducing cross coupling between orthogonal polarizations in satellite communication systems. In transmission, the linearly polarized transmitted waves experience changes in their polarizations due to polarization rotation and polarization conversion effects of the transmission channel especially in the ionosphere. The patented arrangement used microwave components having fixed characteristics which transform the two varying elliptically polarized waves into replicas of the rotated transmitted waves and then a conventional polarization rotator to align the waves in the originally transmitted directions.
An article "Depolarization Properties of Offset Reflector Antennas" by T. Chu et al in IEEE Transactions on Antennas and Propagation, Vol. AP-21, May 1973 at pp. 339-345 discloses and develops the relationships regarding cross-polarization components which are introduced by an offset curved focusing main reflector.
An article "A Dual-Polarized Cylindrical-Reflector Antenna for Communication Satellites" by E. J. Wilkinson in Microwave Journal, Vol. 16, December 1973 at pp. 27-30 and 62 discloses an antenna arrangement with low cross polarization. The Wilkinson arrangement includes two orthogonally-polarized feedhorns and a flat polarized ground plane. Rays from the vertically-polarized line source located above the ground plane appear to come from the cylindrical reflector's actual focal line located below the ground plane. The horizontally-polarized line source is placed on the focal line itself and radiates through the polarized ground plane unaffectedly. The article, however, states that the grating did not change the cross-polarization levels for the vertically-polarized source and it deteriorated the cross-polarization levels for the horizontally-polarized source.
An article "Quasi-Optical Polarization Diplexing of Microwaves" by T. Chu et al in The Bell System Technical Journal, Vol. 54, December 1975 at pp. 1665-1680 relates to avoiding cross polarization in the feed pattern that illuminates antennas. The quasi-optical diplexer cleans up the two orthogonal polarizations simultaneously just before illuminating the subreflector of an antenna having a main reflector and a subreflector.